1. Field of the Invention
The present invention relates to a surface acoustic wave (hereinafter referred to as SAW) filter which has a third interdigital transducer to control a displacement state of a vibration mode in a multi-longitudinal-mode-coupled resonator type SAW filter that utilizes the surface acoustic wave.
2. Description of Related Art
The construction of a conventional multi-longitudinal-mode-coupled resonator type SAW filter having two interdigital transducers (hereinafter referred to as IDTs) is disclosed in Japanese Unexamined Patent Publication No. 61-285814, and a conventional multi-longitudinal-mode-coupled resonator type SAW filter having three IDTs is disclosed in Japanese Unexamined Patent Publication No. 1-231417.
These conventional arts have the problem that a plurality of longitudinal resonance modes (respectively simply designated S0, A0, and S1) in use are difficult to independently control to equalize resonance amplitudes in two or three resonance modes and to set a passband width to a desired width.
The present invention is intended to resolve such a problem, and it is an object of the present invention to realize a multi-longitudinal-mode-coupled resonator type SAW filter that permits a passband width to be relatively easily set, and has flat transmission characteristics with equalized insertion losses in the vicinities of upper end frequency and lower end frequency thereof.
A multi-longitudinal-mode-coupled resonator type SAW filter of a first exemplary embodiment includes, on a piezoelectric substrate, a first IDT for exciting a surface acoustic wave, a second IDT for receiving the surface acoustic wave excited by the first IDT, a third IDT, interposed between the first and second IDTs, for controlling the amplitude of the excited surface acoustic wave, and a pair of reflectors which are arranged with the first, second and third IDTs therebetween in the direction of propagation of the surface acoustic wave,
wherein the reflectors, the first IDT, the second IDT and the third IDT are formed of parallel metallic conductors that are periodically arranged on the piezoelectric substrate,
the distance between the reflector and the closet parallel conductor of the first IDT is set to be equal to the width of a spacing of the first IDT with one period length thereof having alternating spacing and line, the distance between the reflector and the closest parallel conductor of the second IDT is set to be equal to the width of a spacing of the second IDT with one period length thereof having alternating spacing and line,
each of the period length PT1of the parallel conductors of the first IDT and the period length PT2 (=PT1) of the parallel conductors of the second IDT is set to be shorter than the period length PT3 of the parallel conductors of the third IDT (PT3  greater than PT1, PT2), each of the period lengths PT1 and PT2 is set to be shorter than the period length PR of the parallel conductors of the reflectors,
the first, second and third IDTs have a total reflective coefficient xcex93 set to be 10 greater than xcex93 greater than 0.8, and are energy trapped type resonators, and
a two-longitudinal-mode-coupled resonator filter is formed of a fundamental wave symmetrically longitudinal mode S0 which has a displacement amplitude function generally symmetrical with respect to a central position in the direction of propagation X in which the surface acoustic wave is standing, and a fundamental wave anti-symmetric mode A0 which has a displacement amplitude function generally anti-symmetrical with respect to the central position.
In accordance with the first exemplary embodiment, the third IDT is arranged between the first and second IDTs, which are input and output electrodes of the filter, and the IDTs have their own frequencies independently set therein. In this arrangement, an insertion loss and frequency locations of two independent natural vibration modes S0 and A0 which vibrate in the longitudinal direction in a standing wave fashion are controllable.
With the relationship of PT1, PT2, and PT3 set in accordance with the first exemplary embodiment, the insertion loss of the A0 mode is reduced, thereby equalizing the insertion losses of both modes. A two-longitudinal-mode-coupled resonator filter having flat transmission characteristics and a bandwidth of 1000 to 1500 ppm results.
In accordance with the first exemplary embodiment, (PRxe2x88x92PT1)/PR=1xcex5 to 1.7xcex5 and (PRxe2x88x92PT3)/PR=0 to 0.8xcex93 hold where xcex93 is a deviation (fROxe2x88x92fTO)/fRO between a center frequency fROand a frequency fTO where in the setting of the period lengths PT1, PT2, and PT3, fRO is the reflective wave center frequency of the reflector, and fTO is the frequency at which the first IDT, the second IDT, and the third IDT, when considered as a unit, exhibit a maximum reflective conductance G.
With these ranges set, the filter increases energy trapped in the longitudinal direction in the A0 mode having a peak displacement amplitude in the vicinity of the center of each of IDT 1 and IDT 2, and reduces the insertion loss, thereby equalizing the insertion loss to the insertion loss of the S0 mode. A two-longitudinal-mode-coupled resonator filter having flat transmission characteristics and a bandwidth of 1000 to 1500 ppm results.
In accordance with the first exemplary embodiment, the sum of pairs of electrode fingers M=M1+M2+M3 falls within a range from 140 to 180 where M1 is the number of pairs of electrode fingers of the first IDT, M2 is the number of pairs of electrode fingers of the second IDT, and M3 is the number of pairs of electrode fingers of the third IDT, M1 is set to be equal to M2, and DIV falls within a range from 2.1 to 2.4 where DIV is defined as M1=M/DIV.
The number of pairs of the electrode fingers of each IDT is set in this way, and a frequency difference between the S0 mode and the A0 mode is set to within a range from 1000 to 1500 ppm. A two-longitudinal-mode-coupled resonator filter having this bandwidth results.
In accordance with the first exemplary embodiment, a cross bus bar conductor 1 and a cross bus bar conductor 2, each connected to ground potential, are respectively arranged between the first IDT and the third IDT, and between the second IDT and the third IDT, the total width D1 of the width of the cross bus bar conductor 1 and the widths of the spacings on both sides of the cross bus bar conductor 1 is nxcex+(xc2xc)xcex or nxcex+(xc2xe)xcex(n=0, 1, 2, . . . ) where xcex represents the wavelength of the surface acoustic wave, the total width D2 of the width of the cross bus bar conductor 2 and the widths of the spacings on both sides of the cross bus bar conductor 2 is nxcex+(xc2xc)xcex or nxcex+(xc2xe)xcex (n=0, 1, 2, . . . ) where xcex represents the wavelength of the surface acoustic wave, and each of D1 and D2 is within a range of 20 to 100 xcexcm or within a range of 2xcex to 3xcex.
There are times when an unknown vibration mode in a frequency region higher than that of the S0 mode is generated with resonance modes not limited to the S0 and A0 modes, depending on the values of D1 and D2.
Such an unknown vibration mode is not generated if D1 and D2 are set up as described above, and a regular two-longitudinal-mode-coupled resonator filter constructed of S0 and A0 results. If each of D1 and D2 falls within a range from 20 to 100 xcexcm or within a range from 2xcex to 3xcex, the realization of a passband of 1000 to 1500 ppm is not affected.
A multi-longitudinal-mode-coupled resonator type SAW filter of a second exemplary embodiment in this application includes, on a piezoelectric substrate, a first IDT for exciting a surface acoustic wave, a second IDT for receiving the surface acoustic wave excited by the first IDT, a third IDT, interposed between the first and second IDTs, for controlling the amplitude of the excited surface acoustic wave, and a pair of reflectors which are arranged with the first, second and third IDTs interposed therebetween in the direction of the propagation of the surface acoustic wave (in the longitudinal direction X),
wherein the reflectors, the first IDT, the second IDT and the third IDT are formed of parallel metallic conductors that are periodically arranged on the piezoelectric substrate,
the distance between the reflector and the closet parallel conductor of the first IDT is set to be equal to the width of a spacing of the first IDT with one period length thereof having alternating spacing and line, the distance between the reflector and the closest parallel conductor of the second IDT is set to be equal to the width of a spacing of the second IDT with one period length thereof having alternating spacing and line,
each of the period length PT1 of the parallel conductors of the first IDT and the period length PT2(=PT1) of the parallel conductors of the second IDT is set to be longer than the period length PT3 of the parallel conductors of the third IDT (PT3 less than PT1, PT2), each of the period lengths PT1 and PT2 is set to be shorter than the period length PR of the parallel conductors of the reflectors,
the first, second and third IDTs have a total reflective coefficient xcex93 set to be 10 greater than xcex93 greater than 0.8, and are energy trapped type resonators, and
a three-longitudinal-mode-coupled resonator filter is formed of a fundamental wave symmetrically longitudinal mode S0 which has a displacement amplitude function generally symmetrical with respect to a central position in the direction of propagation X in which the surface acoustic wave is standing, a fundamental wave anti-symmetric mode A0 which has a displacement amplitude function generally anti-symmetrical with respect to the central position, and a primary symmetrically longitudinal mode S1 having two nodes in a vibration displacement amplitude and is generally symmetrical with respect to the central position.
The third IDT is arranged between the first and second IDTs, which are input and output electrodes of the filter, and the IDTs have their own frequencies independently set therein. In this arrangement, insertion losses and frequency locations of two independent natural vibration modes S0 and A0 which vibrate in the longitudinal direction in a standing wave fashion, and further the S1 mode are controllable.
With the relationship of PT1, PT2, and PT3 set in accordance with the second embodiment, the insertion losses of the S1 and A0 modes are reduced, the insertion losses are made substantially equal to that of S0 mode. Flat transmission characteristics are thus achieved. Since a frequency difference between S0 and A0 and a frequency difference between A0 and S1 are equalized, filter impedances are also equalized. A three-longitudinal-mode-coupled resonator filter having small ripples thus results.
In accordance with the second exemplary embodiment, (PRxe2x88x92PT1)/PR=0.7xcex5 to 1.2xcex5 and (PRxe2x88x92PT3)/PR=0.9xcex5 to 1.5xcex5 hold where xcex5 is a deviation (fROxe2x88x92fTO)/fRO between a center frequency fRO and a frequency fTO where in the setting of the period lengths PT1, PT2, and PT3, fRO is the reflective wave center frequency of the reflector, and fTO is the frequency at which the first IDT, the second IDT, and the third IDT, when considered as a unit, exhibit a maximum reflective conductance G.
With the relationship of PT1, PT2, and PT3 set as described above, the insertion losses of the S1 and A0 modes are reduced, and the insertion losses are set to be substantially equal to that of S0 mode. Flat transmission characteristics are thus achieved. Since a frequency difference between S0 and A0 and a frequency difference between A0 and S1 are equalized, filter impedances are also equalized. A three-longitudinal-mode-coupled resonator filter having small ripples thus results.
In accordance with the second exemplary embodiment, the sum of pairs of electrode fingers M=M1+M2+M3 falls within a range from 200 to 300 where M1 is the number of pairs of electrode fingers of the first IDT, M2 is the number of pairs of electrode fingers of the second IDT M2, and M3 is the number of pairs of electrode fingers of the third IDT, M1 is set to be equal to M2, and DIV falls within a range from 2.1 to 2.4 where DIV is defined as M1=M/DIV.
The number of pairs of the electrode fingers of each IDT is set in this way, and a frequency difference between the S0 mode and the S1 mode is set to within a range from 1800 to 2000 ppm. A three-longitudinal-mode-coupled resonator filter having this bandwidth results.
In accordance with the second exemplary embodiment, a cross bus bar conductor 1 and a cross bus bar conductor 2, each connected to ground potential, are respectively arranged between the first IDT and the third IDT, and between the second IDT and the third IDT, the total width D1 of the width of the cross bus bar conductor 1 and the widths of the spacings on both sides of the cross bus bar conductor 1 is nxcex+(xc2xc)xcex or nxcex+(xc2xe)xcex (n=0, 1, 2, . . . ) where xcex represents the wavelength of the surface acoustic wave, the total width D2 of the width of the cross bus bar conductor 2 and the widths of the spacings on both sides of the cross bus bar conductor 2 is nxcex+(xc2xc)xcex or nxcex+(xc2xe)xcex (n=0, 1, 2, . . . ) where xcex represents the wavelength of the surface acoustic wave, and each of D1 and D2 is within a range of 20 to 100 xcexcm or within a range of 2xcex to 3xcex.
There are times when an unknown vibration mode in a frequency region higher than that of the S0 mode is generated with resonance modes not limited to the three modes S0, A0 and S1, depending on the values of D1 and D2.
Such an unknown vibration mode is not generated if D1 and D2 are set up as described above, and a regular three-longitudinal-mode-coupled resonator filter constructed of S0, A0, and S1 results. If each of D1 and D2 falls within a range from 20 to 100 xcexcm or within a range from 2xcex to 3xcex, the realization of a passband of 1500 to 2500 ppm is not affected.
In accordance with the first and second exemplary embodiments, the function Wc(X), setting a finger cross width of the first IDT and the second IDT, is determined by equation Wc(X)=A cos (k0Y) where X and Y are related by the following equation A cos (k0Y)=absolute value of (Xxe2x88x92X0) and where A is an arbitrary constant, k0 is a wave number of a fundamental wave mode, a displacement function "psgr"0(Y) of the fundamental wave mode in a transversely inharmonic mode is "psgr"0(Y)=A cos (k0Y), and X0 represents the central position of the first IDT or the second IDT in the X axis direction.
If the displacement distribution of the fundamental S0 mode in the transverse direction is cos (kY), the excitation efficiency for the group of harmonic modes, each having at least one node in the transverse direction, is set to be zero, and the harmonic waves other than the fundamental wave S0 are suppressed. Since no spurious emissions take place from the transverse harmonic mode in this arrangement, a SAW filter free from amplitude ripples in the passband results.
Furthermore, the finger cross width Wc(X) of the first IDT and the second IDT is formed by separating a group of electrode fingers connected between positive and negative feeder conductors of the first IDT and the second IDT, by a spacing equal to or narrower than the xc2xc wavelength of the surface acoustic wave.
As the width of the separation between electrode fingers is decreased, the distortion of the displacement distribution in the transverse direction is reduced, and the displacement relationship between the ideal fundamental wave and the harmonic mode is maintained. The excitation efficiency of the transverse harmonic mode group can be set to be zero. By separating the group of the electrode fingers by the spacing equal to or narrower than the xc2xc wavelength, the spurious emission of the transverse harmonic mode group is suppressed, and a SAW filter free from amplitude ripples in the passband results.
In accordance with the first and second exemplary embodiments, the third IDT has a pattern that is divided and separated at the central position in the direction of propagation of X.
The A0 mode has, at the central position in the X axis, a vibration displacement serving as a node. For this reason, if the third IDT is not patterned to be divided and separated at the central position in the direction of propagation of X, positive and negative charges neutralize each other, liberating Joule heat in energy loss, reducing the Q factor, and thereby increasing the insertion loss in the A0 mode. By patterning the third IDT to be divided and separated at the central position in the direction of propagation of X, the increase in the insertion loss in the A0 mode is prevented, and the ripples in the filter passband are eliminated.
In accordance with the first and second exemplary embodiments, the third IDT has the positive and negative electrode fingers, one of which is connected to neither of the feeder conductors on both sides thereof
In each mode, a vibration state alternates at each electrode finger. When both positive and negative electrode fingers are connected to the feeder conductors, the positive and negative charges neutralize each other in the third IDT, liberating Joule heat in energy loss, reducing the Q factor, and thereby increasing the insertion loss. By leaving one of the positive and negative electrode fingers unconnected to the feeder conductor, the increase in the insertion loss is prevented, and the ripples in the filter passband are eliminated.
In accordance with the first and second exemplary embodiments, the piezoelectric substrate is an ST cut crystal.
The ST cut crystal is excellent in frequency-temperature characteristics and provides a high Q factor in the material of the substrate, thereby making a SAW filter having a high S/N ratio.